Location determination systems determine the location of receivers using data broadcast by satellites. One constellation of Satellites is the Global Positioning System (GPS) operated by the U.S. Air Force. The GPS consists of a constellation of 24 orbiting Satellites which transmit ephemerides via microwave radio.
Position determination systems determine position by receiving the ephemerides from four or more satellites. The ephemerides from each satellite are analyzed in order to determine the apparent distance from the position determination system to each satellite. The determination of apparent distance is made by measuring the time it takes for the signals to travel from each satellite to the receiver of the position determination system. These apparent distances are referred to as pseudoranges.
Pseudoranges are calculated by measuring the time it takes for the signal to travel from the satellite to the receiver. The satellites mark their transmission digitally and the receiver compares the time it receives the time mark with its own time clock. The time delay, referred to as transit time, is typically in the range of about 70-90 milliseconds. Distance is then determined by multiplying transit time by the speed of radio transmissions (approximately 300,000,000 meters/second).
Since the ephemerides data includes the location of each satellite, position may be determined by a geometric calculation that uses the known satellite positions and calculated distances (pseudoranges). GPS based positions are calculated using the World Geodetic System of 1984 (WGS84) coordinate system. These positions are expressed in Earth Centered Earth Fixed (ECEF) coordinates of X, Y, and Z axis. These positions are often transformed into Latitude, Longitude, and Height relative to the WGS84 elipsoid.
However, since the speed of radio transmissions is so fast, any errors in the measured time delay translate into significant errors in the distance calculations. For example, a one millisecond clock error in the receiver will result in a 300 kilometer error in the distance calculations. However, when four satellites are used, errors in the clock in the receiver may be compensated for by applying known calculations. Clock errors in the satellites are closely monitored by a Master Control Station which calculates clock errors in satellites and which uploads clock error data or "clock drift" to each satellite. This clock error data is included in the ephemerides broadcast by each satellite. The master control station also monitors the actual position of each satellite and uploads this information into each satellites ephemerides. Using the clock error data and the ephemerides (position parameters) from each satellite, GPS receiver can calculate an autonomous position.
Errors in location also occur due to intentional introduction of error into the broadcasted ephemerides by the U.S. Air Force and due to atmospheric conditions (referred to hereinafter as "selective availability" or "S/A"). The GPS navigation signals commonly available to civilian users are referred to as the standard positioning service (SPS). The accuracy of SPS is currently specified by the Department of Defense (DOD) to be within 100 meters horizontal position accuracy 95% of the time and 300 meters 99.99% of the time. Though the specified horizontal accuracy may be adequate for some navigational applications such as navigation of a vessel in the open ocean, maritime in coastal waterways often requires an increased level of accuracy as often does the guidance or control of land based machinery in mining, agriculture and/or construction operations.
Errors also result from atmospheric conditions. One method for obtaining accurate position that compensates for intentionally induced error and error due to atmospheric conditions is Differential GPS (DGPS). DGPS systems receive correction data broadcast from a DGPS reference station. A DGPS reference station is located at a fixed and known location. By using this information combined with the satellites' broadcast ephemerides, an actual range to each satellite is able to be determined. By differencing the received range measurement (pseudorange) with this calculated range, a correction to the satellite signal can be broadcast to other receivers that are attempting to solve for their own local location. This correction includes all induced satellite clock errors and atmospheric (ionosphere, troposphere) errors.
Differential GPS systems typically determine position in one of two ways. Traditionally, positions have been calculated using code phase differential techniques. These are normally referred to as DGPS. More recently, carrier phase techniques have been used to determine position. These systems are referred to as Real Time Kinematic (RTK) systems. Both of these techniques use differencing to solve for position, thus being "differential" in nature. The actual techniques and data used is different for each method. Systems that employ RTK techniques and data provide for a more accurate position determination, provided that the differential separation distance is short. For greater distances, a system employing DGPS techniques and data may provide for a more accurate position determination. Therefore, for a GPS system determining position using RTK techniques with data (carrier phase measurements or corrections) from a GPS reference station that is far away, a more accurate position may be determined using DGPS techniques and data (code phase data and corrections) from a closer GPS reference station, provided that such capabilities and data exist.
Differential GPS reference stations may be dedicated facilities with permanent and/or extensive broadcast capabilities or may be simply a transient Differential GPS receiver with data transmitter located at a known location.
Differential GPS reference stations then transmit either their calculated corrections to the GPS signals or their actual observations of the GPS signals (raw data), or both. When transmitting calculated corrections, errors due to atmospheric (troposphere, ionosphere, etc.) and errors due to satellite timing/clock (both intentional and process noise) are represented by the correction value. The application of these corrections at a Differential GPS receiver will compensate for these error sources. Only the common error components are eliminated. A significant area of localized error is GPS signal reflections or refractions, such that the direct signal is not solely identifiable. This is referred to as multi-path effects.
Differential GPS reference stations may also transmit their observations of the GPS signals for each satellite. This method of transmission is popular with RTK positioning techniques and systems due to the nature of typical RTK processing methods. When using this type of data format, errors associated with atmospherics and satellite timing/clock errors are removed at the moving/roving/differential GPS receiver. Most manufactures of RTK systems typically broadcast this data in a format unique to the particular manufacture. For example, Trimble Navigation broadcasts data for use in differential positioning in a Compact Measurement Record (CMR) format. This data contains code phase and carrier phase observable information for both L1 and L2 frequency bands for each satellite. The data is not corrected for S/A or for atmospheric conditions. The data is used primarily for use in RTK positioning.
Other sources of correction data which includes correction data for S/A and atmospheric conditions include broadcasts which conform to the Radio Technical Commission for Maritime Services (RTCM) format. The RTCM has established standards describing format standards, communication bands, and messages for a differential correction GPS service. Correction data which conforms to the RTCM format is broadcast by the U.S. Coast Guard and others to assist in maritime navigation. The U.S. Coast Guard has regional DGPS reference stations which calculate and broadcast correction data using the RTCM format. The RTCM correction data broadcast by some U.S. Coast Guard DGPS reference stations includes carrier phase observable data while data broadcast by other facilities only includes code phase correction data. However, irrespective of whether the particular U.S. Coast Guard facility broadcasts carrier phase data or code phase correction data, the broadcast is typically in a standard RTCM format. Other agencies and port authorities throughout the world broadcast differential correction signals conforming to the RTCM format for navigation in and around coastal areas. Both raw observable data and RTCM "correction data" is referred to hereinafter as "correction data" since both forms of data allow for correction to be made to position.
Prior art systems either receive and process RTCM correction data or CMR correction data or correction data which is in another format, unique to a particular manufacturer. Thus, using prior art systems, in order to determine position using different sources having different broadcast formats require a separate position determination system for each different broadcast format. The use of multiple position determination systems is expensive. Moreover, it is difficult to determine which system should be turned on at any particular time to get the best possible position information.
In many areas, multiple sources of correction signals adhering to a given format are available. Thus, the user must evaluate each channel to identify the source and type of data and then compare the position of each source to the known location of the vessel to determine the best source of data at that particular time. Since the absolute location of a moving vessel is often unknown (otherwise GPS would have limited utility), inspection of the solution noise and other statistical estimators must be employed to determine which solution/data source combination is best--or even usable. This process must be repeated using a different GPS receiver to be able to determine the utility of data in other formats. The process of identifying and connecting the differential GPS receiver, and it's associated processing techniques (RTK and/or DGPS), to the different source/data combinations must be performed perpetually. This process is time consuming, difficult, and prone to errors.
What is needed is a way to obtain differential correction data using differential correction signals from sources that broadcast using different formats which will not require the purchase and operation of separate systems for each different format. In addition, a GPS system is needed which will determine which source of differential correction data provides the best correction data for a particular use. Furthermore, a GPS system which is easy to use and operate is required.